1. Field of the Invention
The present invention relates to a curable composition for use as an electrically-insulating layer for an electric cable, such that the electric cable presents improved ability to withstand aging in moist conditions and when powered.
The invention applies typically, but not exclusively, to the field of medium voltage power cables, in particular for voltages in the range 6 kilovolts (kV) to 60 kV, and to high voltage power cables, in particular at voltages greater than 60 kV, whether used with direct current (DC) or alternating current (AC).
2. Description of Related Art
Medium and high voltage power cables can come into contact with surrounding moisture during their lifetime. The presence of moisture in combination with the presence of an electric field and a polymeric material encourages progressive degradation of the insulating properties of the cable.
This degradation mechanism, known as “water treeing growth” can lead to breakdown of the cable in question and constitutes a considerable threat for the reliability of the power transport network, with the well-known economic consequences caused by electricity failures.
Document EP 0 223 180 describes a composition for use as an electrically-insulating layer in an electric cable for the purpose of limiting water treeing. That composition comprises a copolymer of an oligomer that is used as an agent for reducing water treeing, such as an/oligomer based on an ethylene and an alpha-unsaturated ester, together with a polymeric material that may be a polyethylene, but that could also be a polyvinyl acetal. The quantity of polymeric material, in particular of low density polyethylene, that is used in the examples is at least 95% by weight in the composition, with the remainder of the composition being constituted by the oligomer.
Nevertheless, although that composition does indeed reduce water treeing, it is far from being optimized for the purpose of significantly limiting the degradation associated with water treeing.